Various types of systems for protecting direct current (DC) power distribution systems are known in the art. In one basic type of prior art system, which is exemplified by the system disclosed in U.S. Pat. No. 3,581,152 (Hunt), the direct current operating conditions of the distribution system being protected are monitored by voltage sensitive and/or current sensitive devices.
In another basic type of prior art system, which is exemplified by the systems disclosed in CERCHAR publication No. 1306, dated January, 1963 and published by an agency of the French government, and by the system disclosed in U.S. Pat. No. 3,946,279 (Paice et al), an alternating current (AC) signal is superimposed on the DC power signal, and faults are detected by monitoring the resultant alternating current. Since normal loads have substantial impedance at high frequencies (such as 3 KHz) and draw only a small high frequency current, whereas illegitimate loads such as arcing faults draw a substantial high frequency current, the presence of illegitimate loads are detected in such systems by the high frequency current they draw. However, the substantial inductance of mine power distribution systems makes it difficult for such protection systems to detect faults at points remote from the point at which the AC signal is superimposed on the distribution system. Attempts to overcome this problem have included (1) superimposing the AC signal at more points on the distribution systems, which has the disadvantage of requiring more oscillators and detectors; (2) reducing the impedance of the power transmission lines by running pilot wires with series capacitance parallel to the transmission lines, which is costly and difficult to implement effectively; and (3) increasing the ac impedance of legitimate loads, as exemplified by the Paice et al system, which requires significant quantities of vehicle mounted equipment, and is ineffective if the tranmission lines have a higher than normal impedance, as, for example, is caused by poor grounding or the like.
Applicant is also aware of two systems for detecting balanced ground faults in floating, or ungrounded, direct current electrical systems which employ voltage detectors. These systems are disclosed in U.S. Pat. Nos. 3,754,221 (Stetler) and 3,975,663 (Moorey). In the Stetler system, a resistance connected to each side of the floating circuit is also connected via a two-way switch to a grounded voltage detector, and the voltage drop across each resistance is measured alternatiely to detect faults. In the Moorey system, faults are detected by alternately applying first and second dc voltages having predetermined magnitudes to a ground terminal of the floating circuit which is connected to either side of an electrical supply for the floating circuit, and detecting changes in the magnitudes of the resulting voltages between the electrical supply and the ground terminal. However, in addition to requiring relatively complicated circuitry for their implementation, such systems are not readily adapted to protecting grounded power distribution systems.